Mesoscale convection from a large-scale perspective

This essay concerns precipitating convective cloud systems and convectively-driven mesoscale circulations (“mesoscale convection”) and their role in the large-scale structure of the atmosphere. Mesoscale convection is an important and ubiquitous process on scales of motion spanning a few kilometers to many hundreds of kilometers. It plays a role in the input of energy to the climate system through the radiative effect of upper-tropospheric cloud and water vapor, and enhanced surface fluxes. This is in addition to its important effect on energy, heat and momentum transport within the atmosphere. However, mesoscale convection is neither parameterized nor adequately resolved in atmospheric general circulation models. Its representation in mean-flow terms raises issues that are quite distinct from classical approaches to sub-grid scale convection parameterization.Cloud-resolving modeling and theoretical concepts pertinent to the transport properties and mean-flow effects of organized convection are summarized, as are the main convective parameterization techniques used in global models. Two principal themes that are relevant to the representation of organized mesoscale systems are discussed. First, mesoscale transports and their sub-grid scale approximation with emphasis on dynamical approaches. Second, long time-scale modeling of mesoscale cloud systems that involves the collective effect of convection, boundary and surface layers, radiation, microphysics acting under the influence of large-scale forcing.Finally, major research programs that address the role of precipitating convection and mesoscale processes in global models are summarized.     

Current problems in the stratocumulus-topped atmospheric boundary layer

Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.     

Mesoscale Vegetation-Breeze Circulations and Their Impact on Boundary Layer Structures at Night

The impact of well watered mesoscale wheat over mid-latitude arid areas on mesoscale boundary layer structures (MBLS) and climate has been investigated in the study .using a mesoscale biophysical, meteorological model (BM) developed in the current study. The BM is composed of six modules:mesoscale atmospheric module, soil module, vegetation module, snow-atmosphere interaction module, underlying surface meteorology module and subgrid scale flux parameterization module. The six modules constitute an interacting system by supplying boundary conditions to each other.The investigation indicates that a horizontal pressure gradient associated with mesoscale perturbations in temperature and humidity is created during the day, which results from more water transpired from the vegetation canopy (VC) and evaporated from underlying wet soil. Non-classical mesoscale circulations (called as vegetation-breeze) are forced by the pressure perturbations with wind speeds about 5 m / s, flowing from the VC to the adjacent     

Mesoscale Heat Transport Over Complex Terrain By Slope Winds – A Conceptual Model And Numerical Simulations

Vertical heat fluxes induced by mesoscale thermally driven circulations maycontribute significantly to the subgrid-scale fluxes in large-scale models (e.g.,general circulation models). However, they are not considered in these modelsyet. To gain insight into the importance and possible parameterisation of themesoscale flux associated with slope winds, an analytical (conceptual) modelis developed to describe the relationship between the mesoscale heat flux andatmospheric and land-surface characteristics. The analytical model allows usto evaluate the mesoscale flux induced by slope winds from only a few profilemeasurements within a domain. To validate the analytical model the resultingheat flux profiles are compared to profiles of highly resolved wind and temperaturefields obtained by simulations with a mesoscale numerical model.With no or moderate synoptic wind the mesoscale heat flux generated by the slopewind circulation may be as large as, or even larger than, the turbulent fluxes at thesame height. At altitudes lower than the crest of the hills the mesoscale flux is alwayspositive (upward). Generally it causes cooling within the boundary layer and heatingabove. Despite the simplifications made to derive the analytical model, it reproducesthe profiles of the mesoscale flux quite well. According to the analytical model, themesoscale heat flux is governed by the temperature deviation at the slope surface, thedepth of the slope-wind layer, the large-scale lapse rate, and the wavelength of thetopographical features.     

一种提高气候模式对PNA年际变化模拟能力的方法

The climate modeling community has been challenged to develop a method for improving the simulation of the Pacific-North America (PNA) teleconnection pattern in climate models. The accuracy of PNA teleconnection simulation is significantly improved by considering mesoscale convection contributions to sea surface fluxes. The variation in the PNA over the past 22 years was simulated by the Grid Atmospheric Model of IAP LASG version 1.0 (GAMIL1.0) model, which was guided by observational sea surface temperature (SST) from January 1979 to December 2000. Results show that heating in the tropical central-eastern Pacific is simulated more realistically, and sea surface latent heat flux and precipitation anomalies are more similar to the reanalysis data when mesoscale enhancement is considered during the parameterization scheme of sea surface turbulent fluxes in GAMIL1.0. Realistic heating in the tropical central-eastern Pacific in turn significantly improves the simulation of interannual variation and spatial patterns of PNA.     

Numerical Simulation of Fluxes Generated by Inhomogeneities of the Underlying Surface over the Jinta Oasisin Northwestern China

Using land-use types derived from satellite remote sensing data collected by the EOS Moderate Resolution Imaging Spectroradiometer (EOS/MODIS), the mesoscale and turbulent fluxes generated by inhomogeneities of the underlying surface over the Jinta Oasis, northwestern China, were simulated using the Regional Atmospheric Modeling System (RAMS4.4). The results indicate that mesoscale circulation generated by land-surface inhomogeneities over the Jinta Oasis is more important than turbulence. Vertical heat fluxes and water vapor are transported to higher levels by mesoscale circulation. Mesoscale circulation also produces mesoscale synoptic systems and prevents water vapor over the oasis from running off. Mesoscale circulation transports moisture to higher atmospheric levels as the land-surface moisture over the oasis increases, favoring the formation of clouds, which sometimes leads to rainfall. Large-scale wind speed has a significant impact on mesoscale heat fluxes. During the active phase of mesoscale circulation, the stronger large-scale winds are associated with small mesoscale fluxes; however, background wind seems to intensify the turbulent sensible heat flux and turbulent latent heat flux. If the area of oasis is enlarged properly, mesoscale circulation will be able to transport moisture to higher levels, favoring the formation of rainfall in the oasis and protecting its "cold island" effect. The impact of irrigation on rainfall is important, and increasing irrigation across the oasis is necessary to protect the oasis.     

边界层参数化方案在“灰色区域”尺度下的适用性评估

随着数值预报模式分辨率的提高,当模式网格距与含能湍涡的长度尺度相当时,模式动力过程可解析一部分湍流运动,而剩余的湍流运动仍需参数化,此时便产生了湍流参数化的“灰色区域”问题。对传统的PBL（Planetary Boundary Layer）方案在“灰色区域”下的适用性评估,是改进PBL方案以使其能够适应分辨率变化的前提和基础。本研究基于干对流边界层的大涡模拟试验,比较了WRF（Weather Research and Forecast Model）模式中四种常用的边界层参数化方案[YSU（Yonsei University）、MYJ（Mellor-Yamada-Janjic）、MYNN2.5（Mellor-Yamada-Nakanishi-Niino Level 2.5）、MYNN3）]在“灰色区域”尺度下的表现。研究表明,混合层内总热通量对所使用的参数化方案和水平分辨率均不敏感。不同参数化方案中次网格与网格通量的比例表现出对水平网格距不同的依赖性。局地PBL方案（MYJ、MYNN2.5）在混合层内的平均位温随网格距减小而增大,次网格通量随网格距减小而减小,较参考湍流场对次网格通量有所低估。YSU方案的非局地项几乎不随水平格距改变而变化,对次网格通量的表征并未表现出较强的分辨率依赖性,且过强的非局地次网格输送使混合层内温度层结呈弱稳定,抑制了可分辨湍流输送,不易于激发次级环流。MYNN3方案的非局地次网格通量（负梯度输送项）随网格距减小而减小,使其对次网格通量的表征具有较好的分辨率依赖性。PBL方案在“灰色区域”尺度下的适用性与具体分辨率有关。以分辨率500 m为例,四种PBL方案中不存在一种最佳方案,能对边界层的热力结构和湍流统计特征均有准确的描述。     

Isolating mesoscale coupled ocean–atmosphere interactions in the Kuroshio Extension region

The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale.     

北京地区一次强沙尘天气过程的中尺度通量特征

应用大气所325 m铁塔的湍流资料和梯度资料,对2003年3月20～23日影响北京的一次强沙尘天气过程的中尺度通量和湍流通量进行了分析,结果表明:冷空气的影响是从上层开始的.沙尘爆发前,边界层有强逆温存在,动量以中尺度输送为主,湍流很弱.沙尘爆发时,向下的湍流动量通量明显加大.湍流动量通量是沙尘沉降的主要原因,但沙尘爆发前的中尺度过程不能忽略.沙尘爆发前后,感热通量均以小尺度的湍流输送为主,中尺度输送不明显.     

Impact of subgrid scale scheme on topography and landuse for better regional scale simulation of meteorological variables over the western Himalayas

An attempt is made to integrate subgrid scale scheme on the work of Dimri and Ganju (Pure Appl Geophys 167:1–24, 2007) to understand the overall nature of surface heterogeneity and landuse variability along with resolvable finescale micro/meso scale circulation over the Himalayan region, which is having different altitudes and orientations causing prevailing weather conditions to be complex. This region receives large amount of precipitation due to eastward moving low-pressure synoptic weather systems, called western disturbances, during winter season (December, January, February—DJF). Surface heterogeneity and landuse variability of the Himalayan region gives rise to numerous micro/meso scale circulation along with prevailing weather. Therefore, in the present work, a mosaic type parameterization of subgrid scale topography and landuse within a framework of a regional climate model (RegCM3) is extended to study interseasonal variability of surface climate during a winter season (October 1999–March 2000) of the work of Dimri and Ganju (Pure Appl Geophys 167:1–24, 2007). In this scheme, meteorological variables are disaggregated from the coarse grid to the fine grid, land surface calculations are then performed separately for each subgrid cell, and surface fluxes are calculated and reaggregated onto the coarse grid cell for input to the atmospheric model. By doing so, resolvable finescale structures due to surface heterogeneity and landuse variability at coarse grid are subjected to parameterize at regular finescale surface subgrid. Model simulations show that implementation of subgrid scheme presents more realistic simulation of precipitation and surface air temperature. Influence of topographic elevation and valleys is better represented in the scheme. Overall, RegCM3 with subgrid scheme provides more accurate representation of resolvable finescale atmospheric/surface circulations that results in explaining mean variability in a better way.     

‘A Case Study on a Polar Cold Air Outbreak over Fram Strait using a Mesoscale Weather Prediction Model’

The mesoscale weather prediction model ’Lokal-Modell’ (LM) of the Deutscher Wetterdienst is applied to the situation of an Arctic cold air outbreak in the Fram Strait region in April 1998. Observations are available from a flight along 5⁰E carried out during the ARTIST campaign. Initial and time-dependent boundary data for the simulation are taken from a larger scale operational model system. Using the standard configuration of LM, the simulation reproduced the propagation of cold air and the characteristic structure of the atmospheric boundary layer (ABL) in fair agreement with the observations. However, a detailed comparison revealed three basic problems. Firstly, there is evidence that the available data on sea-ice conditions were insufficient approximations to the true state for several reasons. A modification of the sea-ice data towards observations revealed that parts of the discrepancies were due to the original sea-ice data. Secondly, a control run with the model in its standard configuration shows an insufficient warming of the ABL downstream of the ice edge due to underestimation of surface heat fluxes. A simple modification of the approach for the scalar roughness length resulted in the strongest benefit, while comparative studies showed only a slight sensitivity to different types of parametrisation of turbulent mixing or the inclusion of an additional moist convection parametrisation. Thirdly, in all the simulations the deepening of the convective ABL downstream of the ice edge is weaker than observed. This may be partly due to the thermal stratification above the ABL in the analysis data, which is more stable than observed; but it may also be a hint to the fact that processes near the inversion are insufficiently parametrised in mesoscale models with resolutions as used in LM. The simulated cloud layer in the convective ABL is similar to that observed with respect to condensate content, a sharply defined cloud top, a diffuse lower bound, and continuous light precipitation.     

Simulation of surface energy fluxes using high-resolution non-hydrostatic simulations and comparisons with measurements for the LITFASS-2003 experiment

Land-surface heterogeneity effects on the subgrid scale of regional climate and numerical weather prediction models are of vital interest for the energy and mass exchange between the surface and the atmospheric boundary layer. High-resolution numerical model simulations can be used to quantify these effects, and are a tool used to obtain area-averaged surface fluxes over heterogeneous land surfaces. We present high-resolution model simulations for the LITFASS area near Berlin during the LITFASS-2003 experiment, which were carried out using the non-hydrostatic model FOOT3DK of the University of Köln with horizontal resolutions of 1 km and 250 m. The LITFASS-2003 experimental dataset is used for comparison. The screen level quantities show good quality for the simulated pressure, temperature, humidity and wind speed and direction. Averaged over the four week experimental period, simulated surface energy fluxes at land stations show a small bias for the turbulent heat fluxes and an underestimation of the net radiation caused by excessive cloudiness in the simulations. For eight selected days with low cloud amounts, the net radiation bias is close to zero, but the sensible heat flux shows a strong positive bias. Large differences are found for latent heat fluxes over a lake, which are partly due to local effects on the measurements, but an additional problem seems to be the overestimation of the turbulent exchange under stable conditions in the daytime internal boundary layer over the lake. In the area average over the LITFASS area of 20 ×  20 km², again a strong positive bias of 70 W m^?2 for the sensible heat is present. For the low soil moisture conditions during June 2003, the simulation of the turbulent heat fluxes is sensitive to variations in the soil type and its hydrological properties. Under these conditions, the supply of ground water to the lowest soil layer should be accounted for. Different area-averaging methods are tested. The experimental set-up of the LITFASS-2003 experiment is found to be well suited for the computation of area-averaged turbulent heat fluxes.     

Turbulent fluxes over inhomogeneous landscape

Mesoscale surface turbulent fluxes over a complex terrain surrounded by oceans have been investigated using a 3-D numerical mesoscale model, under conditions with and without synoptic flows. The study indicated that under synoptically calm condition, the allocation and intensity of mesoscale surface turbulent fluxes (MSTFs) were greatly impacted by the thermally forced mesoscale circulation (TFMC) over mesoscale heterogeneous landscape. The max-imum values of sensible (Hs) and latent (LE) heat fluxes were located over the convergent zones and considerably im-pacted by the soil wetness (M), but did not depend strongly on the atmospheric background thermal stability (β0). The simulated results suggested that the sensible heat flux was closely proportional to the square of wind speed in the surface layer. By the action of synoptic flow, the allocation of LE was shifted to downwind, its intensity increased.     

GAMIL模式海气湍流通量参数化方案的改进及其对大气环流年际变率模拟效果的影响

通过引人中尺度对流运动对海表湍流通量的贡献,改进了大气环流模式GAMIL1.0的海气湍流通量参数化方案.利用1979年1月至2000年12月的观测海温资料驱动GAMIL1.0模式,研究了海气湍流通量参数化改进对大气环流年际变化模拟效果的影响.结果表明:采用改进的海气湍流通量参数化方案,模拟的热带海表湍流热通量得到增强,...     

Parameterization of the thermal impacts of sub-grid orography on numerical modeling of the surface energy budget over East Asia

Summary A parameterization scheme for the thermal effects of subgrid scale orography is incorporated into a regional climate model (developed at Nanjing University) and its impact on modeling of the surface energy budget over East Asia is evaluated. This scheme includes the effect of terrain slope and orientation on the computation of solar and infrared radiation fluxes at the surface, as well as the surface sensible and latent heat fluxes. Calculations show that subgrid terrain parameters alter the diurnal cycle and horizontal distributions of surface energy budget components. This effect becomes more significant with increased terrain slope, especially in winter. Due to the inclusion of the subgrid topography, the surface area of a model grid box changes over complex terrain areas. Numerical experiments, with and without the subgrid scale topography scheme, show that the parameterization scheme of subgrid scale topography modifies the distribution of the surface energy budget and surface temperature around the Tibetan Plateau. Comparisons with observations indicate that the subgrid topography scheme, implemented in the climate model, reproduces the observed detailed spatial temperature structures at the eastern edge of the Tibetan Plateau and reduces the tendency to overestimate precipitation along the southern coastal areas of China in summer.     

Effects of land-surface heterogeneity on numerical simulations of mesoscale atmospheric boundary layer processes

Landscape heterogeneity that causes surface flux variability plays a very important role in triggering mesoscale atmospheric circulations and convective weather processes. In most mesoscale numerical models, however, subgrid-scale heterogeneity is somewhat smoothed or not adequately accounted for, leading to artificial changes in heterogeneity patterns (e.g., patterns of land cover, land use, terrain, and soil types and soil moisture). At the domain-wide scale, the combination of losses in subgrid-scale heterogeneity from many adjacent grids may artificially produce larger-scale, more homogeneous landscapes. Therefore, increased grid spacing in models may result in increased losses in landscape heterogeneity. Using the Weather Research and Forecasting model in this paper, we design a number of experiments to examine the effects of such artificial changes in heterogeneity patterns on numerical simulations of surface flux exchanges, near-surface meteorological fields, atmospheric planetary boundary layer (PBL) processes, mesoscale circulations, and mesoscale fluxes. Our results indicate that the increased heterogeneity losses in the model lead to substantial, nonlinear changes in temporal evaluations and spatial patterns of PBL dynamic and thermodynamic processes. The decreased heterogeneity favor developments of more organized mesoscale circulations, leading to enhanced mesoscale fluxes and, in turn, the vertical transport of heat and moisture. This effect is more pronounced in the areas with greater surface heterogeneity. Since more homogeneous land-surface characteristics are created in regional models with greater surface grid scales, these artificial mesoscale fluxes may have significant impacts on simulations of larger-scale atmospheric processes.     

A parameterization for modelling the meteorological effects of tall forests — A case study of a large clearing

A parameterization scheme has been developed to describe the effects of a tall forest on the mean structure of the atmospheric boundary layer (ABL). The main advantage of the scheme is that dynamical and thermodynamical effects of a forest surface can be simulated satisfactorily using only a coarse-grid resolution within numerical models. Thereby, the canopy layer is parameterized as a quasi-subgrid phenomenon. This makes it possible to study meteorological phenomena within the ABL in a very economical way (with respect to computational time) whereby, nevertheless, more detailed information concerning the forest surface is taken into account than could be done using the same grid resolution and quite simple assumptions describing the canopy, e.g., the effective roughness.The applicability in numerical models is shown by using a slightly modified two-dimensional version of the mesoscale model FITNAH. For comparison, simulations with a high numerical grid resolution within the canopy have been carried out.Model results reproduce the known meteorological phenomena in forested areas, e.g., a stable thermal stratification near the surface during the day, and at night, a neutral — or slightly unstable condition — and, in general, reduced windspeed within the canopy layer.Diurnal variations and spatial distributions of temperature and humidity are found to be similar for both cases. Also, a thermally-induced local circulation system in the vicinity of a large clearing has been simulated satisfactorily.A comparison of the calculated results verifies that the parameterization scheme is quite suitable for simulating the effects of plant canopies on the distributions of meteorological variables in the ABL.     

The Effects of Strong Ageostrophic Outflows on the Formation of Surface Mesoscale Pressure Systems in Squall Lines

Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind ficld caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic(anticyclonic)circulation in association with a positive(negative)vorticity line.By use of the shallow-water model,the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines.In the case of two vorticity line,one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered,three mesoscale circulations develop,and a mesohigh and two mesolows similar to the thunderstorm high,wake low and pre-squall mesolow of a mature squall line are produced.Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows.The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows,and produce positive-negative-positive arranged vcrtical vorticity bands(VBs)along the direction normal to the squall line,then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment.By use of the scale separation method,this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.